Interactive comment on “ Climatic information of Western Sahel ( 1535 – 1793 AD ) in original documentary sources ” by V . Millán and (original) (raw)
Related papers
Sahel droughts and Enso dynamics
Geophysical Research Letters, 1996
Correlations between summer Sahel rainfall and Southern Oscillation Index has increased during the last thirty years. At high frequency time scale (periods lower than 8 years), an intertropical Atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature (SST) anomalies between the eastern equatorial parts of Pacific and Atlantic. This zonal connection worked well during most of the E1 Nino/Southern Oscillation (ENSO) events occurring after 1970; positive/negative SST anomalies in the eastern Pacific/Atlantic led to rainfall deficits over the whole West Africa. At low frequency time scale (periods greater than 8 years), positive SST anomalies in the Indian ocean and in equatorial Pacific existing after 1970 have been associated with decreasing rainfall intensity over West Africa through another zonal divergent circulation. These different time scales remote SST forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between Sahel drought and ENSO dynamics after 1970.
Modern drought conditions in western Sahel unprecedented in the past 1600 years
As climate model uncertainties remain very large for future rainfall in the Sahel, a multi-centennial perspective is required to assess the situation of current Sahel climate in the context of global warming. We present here the first record of hydrocli-matic variability over the past 1600 years in Senegal, obtained from stable oxygen isotope analyses (δ 18 O) in archaeological shell middens from the Saloum Delta. During the preindustrial period, the region was relatively humid, with maximum humidity reached during the period from AD 1500 to AD 1800, referred to as the Little Ice Age. A significant negative link is observed at the centennial scale between global temperature and humidity in the Sahel that is at odds with the expected effects of latitudinal shifts of the intertropical convergence zone during the last millennium. In the context of the past 1600 years, the Western Sahel appears to be experiencing today unprecedented drought conditions. The rapid aridification that started ca. AD 1800 and the recent emergence of Sahel drought from the natural variability point to an anthropogenic forcing of Sahel drying trend. This new long-term perspective suggests that the recovery of Sahel rainfall in the last decade may only result from short-term internal variability, and supports climate models that predict an increase of Sahel drought under future greenhouse climate.
We propose a re-interpretation of the oceanic influence on the climate of the African Sahel that is consistent across observations, 20th century simulations and 21st century projections, and that resolves the uncertainty in projections of precipitation change in this region: continued warming of the global tropical oceans increases the threshold for convection, potentially drying tropical land, but this 'upped ante' can be met if sufficient moisture is supplied in monsoon flow. In this framework, the reversal to warming of the subtropical North Atlantic, which is now out-pacing warming of the global tropical oceans, provides that moisture, and explains the partial recovery in precipitation since persistent drought in the 1970s and 1980s. We find this recovery to result from increases in daily rainfall intensity, rather than in frequency, most evidently so in Senegal, the westernmost among the three Sahelian countries analyzed. Continuation of these observed trends is consistent with projections for an overall wetter Sahel, but more variable precipitation on all time scales, from intra-seasonal to multi-decadal.
This study uses a range of published and unpublished historical documentary sources to explore the nature of rainfall variability in the Kalahari Desert and adjacent hardveld regions of central southern Africa during the seventeen Pacific El Niño-Southern Oscillation (ENSO) episodes that occurred between 1840 and 1900. Documentary data are used in two ways. First, maps of relative annual rainfall levels are presented for each of the 12 single-year and five protracted ENSO episodes during the period, in order to identify the associated inter-annual rainfall variations. These suggest that the relationship between ENSO episodes and rainfall variability identified for the twentieth century, whereby warm events are frequently preceded by wetter conditions during the austral summer prior to the event year and succeeded by drought in the following summer, has broadly held for much of the last 160 years. This is despite the long-term fluctuations in precipitation and temperature which are known to have occurred over this period. Droughts are identified following at least thirteen of the 17 single-year and protracted ENSO episodes. Pre-ENSO wetter periods are less common, with only nine of the ENSO episodes preceded by abovenormal rainfall. Second, the documentary data are analyzed in detail in order to reveal any evidence for high resolution intra-annual variations in the seasonal distribution of rainfall during ENSO events. Seasonal sequences of rainfall/drought appear to have closely followed contemporary patterns, with heavy rainfall commonly occurring late in the pre-ENSO year or early in the ENSO year(s), and drought at the start of the post-ENSO year. This relationship can be seen to hold most strongly for single-year ENSO warm events and for the first year of protracted events, but rainfall conditions were more variable during the later years of protracted events.
Evolution of some observed climate extremes in the West African Sahel
Weather and Climate Extremes, 2013
Climate variability and change affect most socioeconomic sectors in West Africa. It is now admitted that the variability of climate has increased since the 1950s mainly because of the increased concentration of anthropogenic greenhouse gases in the atmosphere. In this study, we analyze the evolution of some extreme temperature and precipitation indices over a large area of West Africa spanning from latitudes 10-251N and longitudes 171W-151E. The results show a general warming trend throughout the region during the period from 1960 to 2010, namely through a negative trend in the number of cool nights, and more frequent warm days and warm spells. This was the case not only for locations inside the continent, but also for those in coastal areas. Trends in rainfall related indices are not as uniform as the ones in temperatures. Nevertheless, a general tendency of decreased annual total rainfall and maximum number of consecutive wet days characterizes the study period. The cumulated rainfall of extremely wet days shows a positive trend in most locations. As for the maximum number of consecutive wet days, it shows an overall decreasing trend from 1960 to the mid 1980s, but starting from the late 1980s, an increasing trend is observed in several locations, indicating that extreme rainfall events have become more frequent in the West African Sahel during the last decade, compared to the 1961-1990 period. Policy implications of these observed trends may include investment and promotion of low cost and environmentally friendly energy production systems, the redesign of infrastructure and production systems to account for higher risks of losses due to floods and/or droughts, and the promotion of research for more heat tolerant crop/animal species and cultivars/breeds.
ENSO signals in South America: rains and floods in the Paraná River region during colonial times
Climatic Change, 2007
Several studies show that the El Niño-Southern Oscillation (ENSO) is an important factor in determining interannual rainfall variability in South America. This signal is detected in the region including Northeastern Argentina, Uruguay and part of southern Brazil, in the form of excessive rains and big floods in the regional rivers. To check that this relationship was similar in the past the objectives of this paper were to construct a time series of large floods in the Paraná River region from documentary records, during the sixteenth to eighteenth centuries, and to evaluate the relationship between that historical record of extreme floods, the ENSO documentary evidence (Ortlieb, The Documented Historical Record of El Niño Events in Perú: An Update of the Quinn Record (Ortlieb, sixteenth through nineteenth centuries, Diaz and Markgraf, (eds.), El Niño and the southern oscillation. Multiscale variability and global and regional Impacts. Cambridge Unive rsity Press, pp. 207–295, 2001; Quinn and Neal, The historical record of El Niñoevents, Bradley and Jones (eds.), Climate since a.d. 1500, Routledges, pp. 623–648, 1992) and the temperature index of the Pacific Ocean (Mann ME et al., Global temperature patterns in past centuries: an interactive presentation, IGBP pages/world data center for paleoclimatology data contribution series #2000-075. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA, 2000). Considering that the period 1904–2000, where 11 out of 16 floods occurred during El Niño events, it can be concluded that the proportion of years with exceptional flow volume in the Paraná River in years with El Niño events in the seventeenth and eighteenth centuries was relatively lower than that of the twentieth century. The reason for this difference is discussed.